The invention relates to a testing and calibrating device for an evaluation circuit for a linear oxygen probe (referred to below as lambda probe or probe) in an internal combustion engine, particularly in a motor vehicle internal combustion engine.
The production process for engine control circuits for internal combustion engines requires that the probe evaluation circuit be tested and calibrated in the fitted state. This requires that all relevant parameters be verified under operating conditions—various supply voltages and temperatures for the engine control circuit, but also various operating states (lambda values) for the probe. It is also necessary to test whether the evaluation circuit identifies particular probe faults.
Furthermore, it is desirable to be able to perform not only the legally required OBD (on board diagnostics) for the probe but also calibration of the system (probe and evaluation circuit) when the internal combustion engine is operating.
The probe and the evaluation circuit are a closed control system. Simple measurement of the electrical properties of the evaluation circuit (for example offset or gain) is therefore not very revealing. The test needs to be performed when the control loop is closed and stable.
Although a test using a connected lambda probe allows measurement in the operating state, it is time-consuming and imprecise. The addition of a fault to demonstrate the diagnostic function is done using switches in the probe supply lines, for example, which allows short circuits and interruptions to be simulated. This is very time-consuming and susceptible to error. In addition, various operating points of the probe can be measured only by altering the oxygen concentration around the probe. This requires a very complex gas-changing device which regularly needs to be calibrated. Since a gas change, for technical reasons, proceeds comparatively slowly, it is not possible to assess the control stability of the system.
The system (probe and evaluation circuit) is calibrated during engine operation at two operating points:
a) when λ=1. In this case, no or only a minimal pump current should flow, since the oxygen concentrations in the exhaust from the internal combustion engine and in the measuring cell are balanced;
b) when λ=∞, that is to say with no fuel, i.e. when a motor vehicle is in overrun mode. In this case, the (maximum) pump current required is measured.
The values obtained for these two measurements can be used to determine the offset and gradient of the transfer function. The arithmetic values are stored in a correction table.
Overall, the method is extremely complex and has only limited reliability (on account of possible residual exhaust or cooling of the probe in overrun mode).